U.S. patent number 5,494,700 [Application Number 08/222,860] was granted by the patent office on 1996-02-27 for method of coating a substrate with a metal oxide film from an aqueous solution comprising a metal cation and a polymerizable organic solvent.
This patent grant is currently assigned to The Curators of the University of Missouri. Invention is credited to Harlan U. Anderson, Chieh-Cheng Chen, Magdi M. Nasrallah.
United States Patent |
5,494,700 |
Anderson , et al. |
February 27, 1996 |
Method of coating a substrate with a metal oxide film from an
aqueous solution comprising a metal cation and a polymerizable
organic solvent
Abstract
A method for preparing a substrate coated with a
polycrystalline, metal oxide film using polymeric precursors. The
oxide films prepared by the method of the present invention are
dense (i.e., substantially free of cracks and pinholes) and may be
used, for example, as an electrolyte or electrode in intermediate
temperature solid oxide fuel cells (SOFCs) or as gas separation
membranes.
Inventors: |
Anderson; Harlan U. (Rolla,
MO), Nasrallah; Magdi M. (Rolla, MO), Chen;
Chieh-Cheng (Rolla, MO) |
Assignee: |
The Curators of the University of
Missouri (Columbia, MO)
|
Family
ID: |
22834020 |
Appl.
No.: |
08/222,860 |
Filed: |
April 5, 1994 |
Current U.S.
Class: |
427/115; 427/240;
427/376.2; 427/384 |
Current CPC
Class: |
B01D
71/024 (20130101); H01M 4/9033 (20130101); B01D
67/0046 (20130101); B01D 53/228 (20130101); C04B
41/4554 (20130101); H01M 8/1253 (20130101); C04B
41/009 (20130101); C04B 41/81 (20130101); C04B
41/4554 (20130101); C04B 41/5027 (20130101); C04B
41/009 (20130101); C04B 35/01 (20130101); C04B
2111/00801 (20130101); C04B 2111/00853 (20130101); Y02P
70/56 (20151101); B01D 2325/04 (20130101); B01D
2323/06 (20130101); Y02P 70/50 (20151101); B01D
67/0083 (20130101); Y02E 60/50 (20130101); Y02E
60/525 (20130101) |
Current International
Class: |
C04B
41/81 (20060101); C04B 41/45 (20060101); B01D
53/22 (20060101); B01D 71/02 (20060101); B01D
71/00 (20060101); H01M 8/12 (20060101); B05D
005/12 () |
Field of
Search: |
;427/115,240,376.2,384 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
G Yi et al, "Sol-Gel Processing of Complex Oxide Films", Ceramic
Bulletin vol. 70 No. 7 1991, pp. 1173-1179. .
J. Reed, "Introduction to the Principles of Ceramic Processing,"
John Wiley & Sons, New York, 1988 pp. 403-405. .
Osamu Yamamoto and Tadashi Sasamoto "Indium tin oxide thin films
prepared by thermal decomposition of ethylene glycol solution."
Materials Research Society, vol. 7, No. 9 (Sep. 1992), pp.
2488-2491. .
C. C. Chen, M. M. Nasrallah, H. U. Anderson, "Synthesis and
Characterization of (CeO.sub.2).sub.0.8 (SmO.sub.1.5).sub.0.2 Thin
Films from Polymeric Precursors", Journal of the Electrochemical
Society vol. 140, No. 12, pp. 3555-3560, Dec. 1993. .
C. C. Chen, M. M. Nasrallah, H. U. Anderson, "Synthesis and
Characterization of YSZ Thin Film Electrolytes", presented Sep.
1993 at the 9th International Conference on Solid State Ionics,
Hague, the Netherlands, pp. 1-18. .
C. C. Chen, M. M. Nasrallah, H. U. Anderson, "Cathode/Electrolyte
Interactions and Their Expected Impact on SOFC Performance", The
Electrochemical Society Proceeding Series, PV-4, pp. 598-612, Apr.
1993. .
C. C. Chen, M. M. Nasrallah, H. U. Anderson, "Preparation and
Electrode Characteristics of Dense La.sub.0.6 Sr.sub.0.2 Fe.sub.0.8
O.sub.3 Thin Film by Polymeric Precursors", The Electrochemical
Society Proceeding Series, PV-4, pp. 252-266, Apr. 1993. .
C. C. Chen, M. M. Nasrallah, H. U. Anderson, "Thin Film
Electrolytes for Intermediate Temperature SOFC Application", pp.
515-518, 1992. .
C. C. Chen, M. M. Nasrallah, H. U. Anderson, "Synthesis and
Characterization of the (CeO.sub.2).sub.0.5 (SmO.sub.1.5).sub.0.2
Thin Films From Precursors", abstract of presentation made Apr.
1992 at the 94th American Ceramic Society Meeting. .
H. U. Anderson, M. M. Nasrallah, F. D. Blum, M. S. Smith,
"Polymeric Synthesis of Perovskite Powders and Films", National
Institute of Standards and Technology, Special Publication 804, pp.
179-184, Jan. 1991. .
H. U. Anderson, C. C. Chen, J. C. Wang, Pennell, "Synthesis of
Conducting Oxide Films and Powers From Polymeric Precursors",
Ceramic Powder Science III, pp. 749-755, 1990. .
C. C. Chen, H. U. Anderson, "Synthesis of YBa.sub.2 Cu.sub.3
O.sub.7-x Superconductor By the Ethylene Diamine Process", Advanced
Advanced Materials Conference II Proceedings, pp. 520-530, 1989.
.
N. C. Eror, H. U. Anderson, "Polymeric Precursor of Ceramic
Materials", Materials Research Society Symposia, Proceedings, vol.
73, pp. 571-577, 1986. .
Bulent E. Yoldas, "Alumina Sol Preparations", Ceramic Bulletin vol.
54, pp. 289-290, 1974..
|
Primary Examiner: Utech; Benjamin
Attorney, Agent or Firm: Senniger, Powers, Leavitt &
Roedel
Claims
What is claimed is:
1. A method for preparing a component for a solid oxide fuel cell
comprising a substrate coated with a polycrystalline metal oxide
film, the oxide film being comprised of an oxide selected from the
group consisting of (CeO.sub.2).sub.1-x (SmO.sub.1.5).sub.x,
(ZrO.sub.2).sub.1-x (YO.sub.1.5).sub.x, La.sub.1-x Sr.sub.x
MnO.sub.3 and La.sub.1-x Sr.sub.x Co.sub.y Fe.sub.1-y O.sub.3,
wherein x and y are between 0 and 1 and are selected to represent
the nominal composition of the oxide film, the method
comprising:
preparing a precipitate-free starting solution by dissolving metal
cation source compounds for each of the metal constituents of the
oxide film in an aqueous mixture comprising a polymerizable organic
solvent, the cations of each metal being present in the starting
solution in a molar ratio corresponding to the nominal composition
of the oxide film;
heating the starting solution to form a polymeric precursor
comprising a polymer containing the metal cations, the precursor
being free of precipitates;
depositing the polymeric precursor onto a surface of the
substrate;
spinning the substrate to thereby coat the substrate with a film of
the polymeric precursor; and
calcining the deposited film of polymeric precursor in the presence
of oxygen and at a temperature not in excess of 600.degree. C. to
convert the film of polymeric precursor into the polycrystalline
metal oxide film, the oxide film having an average grain size less
than about 0.5 .mu.m and being substantially free of cracks or
pinholes.
2. A method for preparing a gas separation membrane for selectively
transferring a component of a gaseous mixture across the membrane,
the membrane comprising a polycrystalline metal oxide film, the
method comprising:
preparing a precipitate-free starting solution by dissolving at
least one metal cation source compound in an aqueous mixture
comprising a polymerizable organic solvent;
heating the starting solution to form a polymeric precursor
comprising a polymer containing the metal cations, the precursor
being free of precipitates;
depositing a film of the polymeric precursor onto a surface of a
porous substrate, the porosity of the substrate being sufficient to
allow transport of the selected component of the gas mixture across
the membrane; and
calcining the deposited film of polymeric precursor in the presence
of oxygen to convert the film of polymeric precursor into the
polycrystalline metal oxide film, the oxide film being
substantially free of cracks or pinholes.
3. The method of claim 2 wherein the porous substrate has an
average pore diameter in excess of about 5 .mu.m.
4. The method of claim 2 wherein the polymerizable organic solvent
is ethylene glycol.
5. The method of claim 2 wherein a pH control agent selected from
the group consisting of nitric acid, citric acid, hydrochloric
acid, glycine, ammonium hydroxide and ethylene diamine is added to
the starting solution to inhibit the formation of precipitates.
6. The method of claim 2 wherein the film of the polymeric
precursor is deposited onto a surface of a porous substrate by
spin-coating the substrate with the polymeric precursor.
7. The method of claim 6 wherein the viscosity of the polymeric
precursor is between about 90 and 190 cP at 25.degree. C.
8. The method of claim 7 wherein the substrate is spun at a rate
between about 1500 and about 3000 rpm.
9. The method of claim 2 wherein the oxide comprises two or more
metal constituents, the cations of each metal being present in the
starting solution in a molar ratio corresponding to the nominal
composition of the oxide film.
10. The method of claim 2 wherein the cation source compound is
selected from the group consisting of carbonates, nitrates,
chlorides and hydroxides of the oxide's metal constituents.
11. The method of claim 2 wherein the deposited film of polymeric
precursor is dried at a temperature not in excess of about
80.degree. C. to substantially remove remaining solvent from the
precursor prior to calcining the coated substrate.
12. The method of claim 2 wherein the deposited film of polymeric
precursor is calcined at a temperature not in excess of 600
.degree. C.
13. The method of claim 12 wherein the oxide film exhibits a
polycrystalline microstructure having an average grain size less
than about 0.5 .mu.m.
14. A method for preparing a gas separation membrane for
selectively transferring oxygen from a gaseous mixture across the
membrane, the membrane comprising a polycrystalline metal oxide
film of (ZrO.sub.2).sub.1-x (YO.sub.1.5).sub.x or La.sub.1-x
Sr.sub.x Co.sub.y Fe.sub.1-y O.sub.3, wherein x and y are between 0
and 1 and are selected to represent the nominal composition of the
oxide, the method comprising:
preparing a precipitate-free starting solution by dissolving source
compounds for cations of Zr and Y or La, Sr, Co and Fe in an
aqueous mixture comprising a polymerizable organic solvent;
heating the starting solution to form a polymeric precursor
comprising a polymer containing the metal cations, the precursor
being free of precipitates;
depositing a film of the polymeric precursor onto a surface of a
porous substrate, the porosity of the substrate being sufficient to
allow transport of oxygen across the substrate/membrane boundary;
and
calcining the deposited film of polymeric precursor in the presence
of oxygen to convert the film of polymeric precursor into the
polycrystalline metal oxide film, the oxide film being
substantially free of cracks or pinholes.
15. The method of claim 14 wherein the porous substrate has an
average pore diameter in excess of about 5 .mu.m.
16. The method of claim 15 wherein the gaseous mixture is air.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a method for preparing thin,
polycrystalline metal oxide films. The films prepared by the method
of the present invention are dense (i.e., substantially free of
cracks and pinholes) and may be used, for example, as an
electrolyte or electrode in intermediate temperature solid oxide
fuel cells (SOFCs) or as gas separation membranes.
A fuel cell is a static device that converts the chemical energy in
a fuel directly, isothermally, and continuously into electrical
energy. Fuel and oxidant (typically oxygen in air) are fed to the
cell in which an electrochemical reaction takes place that oxidizes
the fuel, reduces the oxidant, and releases energy. The energy
released is in both electrical and thermal forms, the electrical
part providing the required output. In a typical power generation
plant employing fuel cells, hydrocarbon fuel (e.g., natural gas) or
gasified coal is reformed first to produce hydrogen-rich and
sulphur-free gas that enters the fuel cell stack where it is
electrochemically "burned" to produce the electrical and thermal
outputs. The electrical output of a fuel cell is low-voltage
high-current dc. By utilizing a properly organized stack of cells
and an inverter, utility-grade ac output is obtained.
Fuel cells generating electricity from natural gas offer
significant advantages over conventional power generation systems
including improved reliability and safety and reduced airborne
emissions. Also, since a fuel cell completely bypasses the
thermal-to-mechanical conversion involved in a conventional power
plant and since its operation is isothermal, fuel cells are not
Carnot-limited. Efficiencies in the range of 43 to 55% are
forecasted for modular dispersed generators featuring fuel cells.
The possibility of using fuel cells in combined heat and power
units provides the cleanest and most efficient energy system option
utilizing valuable natural gas resources.
One of the most promising types of fuel cells being developed are
SOFCs. SOFCs are particularly desirable as alternatives to
conventional power sources due to their reliability, increased
power-to-weight and power-to-volume ratios, simplicity, and
environmental advantage over other types of fuel cells. These
characteristics make SOFCs ideal for use in remote electrical power
generation applications such as in space stations and satellites.
Alternatively, SOFCs can be integrated with a coal gasifier and a
steam bottoming cycle to form a more conventional electrical power
generation system. The reliability of SOFCs is mainly attributed to
stability of the components as well as the presence of low kinetic
barriers at the electrode/electrolyte interfaces.
SOFCs are typically of planar or tubular construction and comprise
layers or films of various polycrystalline metal oxides which form
the electrolyte and electrode components of the fuel cell. The
electrolyte of choice in state-of-the-art SOFCs is a film made from
(ZrO.sub.2).sub.1-x (YO.sub.1.5).sub.x (YSZ), while suitable
electrodes are comprised of films made from La.sub.1-x Sr.sub.x
MnO.sub.3 (LSM) or La.sub.1-x Sr.sub.x Co.sub.y Fe.sub.1-y O.sub.3
( LSCF ). In the preceding formulas, x and y have values between 0
and 1 and can be varied to provide an oxide of the desired nominal
composition.
Conventional SOFCs typically comprise a layer of YSZ electrolyte
40-160 .mu.m thick and require operating temperatures of
approximately 1000.degree. C. to achieve adequate oxygen transport
across the electrolyte. Conventional SOFC fabrication techniques
require even higher temperatures. Reduction in fuel cell operating
and fabrication temperatures will improve cell performance not only
due to reduced interfacial resistance between the electrolyte and
the cathode, but also due to a reduction in other related problems
such as thermal stresses, interdiffusion, sealings and
interconnections.
Reduced operating temperatures can be achieved by developing
methods of preparing thinner metal oxide films for use as
electrolytes and electrodes in SOFCs. A thinner electrolyte, for
example, will provide a shorter path for ion transfer and will
result in the electrolyte exhibiting less ohmic resistance at
reduced temperatures (e.g., 600.degree.-800.degree. C.) as compared
to conventional, thicker electrolytes.
Metal oxide films are also used as gas separation membranes. For
example, a gas separation membrane comprised of a metal oxide film
may be incorporated in an air pump used to separate pure oxygen
from air. In such a device, the oxide film serves as a membrane
which selectively transfers the desired component of the air
mixture (i.e., oxygen) across the membrane.
Various attempts have been made to fabricate thin metal oxide films
using electrochemical vapor deposition (EVD), plasma spraying, RF
sputtering, spray pyrolysis, and sol-gel methods, etc. Among these
various alternative methods, the sol-gel derived oxide films
possess improved homogeneity, higher purity, and offer the
advantage of processing a wide range of oxide compositions at
somewhat lower temperatures as compared to the other synthesis
techniques. However, sol-gel alkoxide precursors are moisture
sensitive and their shelf-life is relatively short. Accordingly, a
need exists for a film preparation method capable of providing high
quality, thin, dense, polycrystalline metal oxide films at even
lower processing temperatures. Dense films are particularly
important in SOFC and gas separation membrane applications. On the
other hand, lower processing temperatures are desirable because
they decrease unwanted thermal interactions and interdiffusion
between the film and the substrate on which the film is deposited
as well as reduce the tendency for cracks and other discontinuities
to form in the film.
SUMMARY OF THE INVENTION
Among the objects of the present invention, therefore, is the
provision of a method for preparing thin, polycrystalline metal
oxide films substantially free of cracks and pinholes; the
provision of such a method wherein the processing temperature is
reduced; and the provision of such a method capable of producing a
wide variety of metal oxide films.
Briefly, therefore, the present invention is directed to a method
for preparing a substrate coated with a polycrystalline metal oxide
film. The method comprises first preparing a precipitate-free
starting solution containing cations of the oxide's metal
constituents dissolved in an aqueous mixture comprising a
polymerizable organic solvent. The pH of the starting solution is
controlled such that the solution remains substantially free of
precipitates upon subsequent heating. The starting solution is then
heated to form a polymeric precursor substantially free of
precipitates. The polymeric precursor comprises a polymer
containing the metal cations. The precursor is deposited on the
substrate and the substrate is then spun to thereby coat the
substrate with a thin film of the polymeric precursor. The
deposited film of polymeric precursor is then calcined in the
presence of oxygen and at a temperature not in excess of
600.degree. C. to convert the film of polymeric precursor into the
polycrystalline metal oxide film. The oxide film is substantially
free of cracks or pinholes and exhibits a polycrystalline
microstructure having a substantially uniform grain morphology and
an average grain size less than about 0.5 .mu.m.
Other objects and features of this invention will be in part
apparent and in part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow chart for the metal oxide film synthesis method of
the present invention.
FIG. 2 is the FTIR transmission spectra of ethylene glycol and the
polymeric precursor for (CeO.sub.2).sub.0.8 (SmO.sub.1.5).sub.0.2
as a function of heating time.
FIG. 3 shows the change in viscosity of the polymeric precursor as
function of starting solution heating time for the CSO oxide film
prepared in Example 1.
FIGS. 4 (A), 4 (B), 4 (C) and 4 (D) show the 3-D atomic force
microscopy (AFM) micrographs for the CSO oxide film deposited on a
Si substrate in Example 1. The as-deposited films were fast fired
for 1 minute at 320.degree., 400.degree., 600.degree., and
800.degree. C., respectively.
FIG. 5 is the cross-section SEM micrograph of a CSO oxide film
deposited on a LSCF substrate as prepared in Example 1 after five
spin-coatings and fast firing at 600.degree. C. for 1 min.
FIG. 6 is the x-ray diffraction patterns showing phase development
of a CSO oxide film deposited on a Pt substrate as prepared in
Example 1.
FIG. 7 is the x-ray diffraction patterns showing phase development
of a CSO oxide film deposited on a LSCF substrate as prepared in
Example 1.
FIG. 8 is a surface Auger electron spectroscope (AES) spectrum of a
CSO oxide film deposited on a Si substrate as prepared in Example
1.
FIG. 9 is an in-depth AES compositional profile for a CSO oxide
film deposited on a Si substrate as prepared in Example 1. The
oxide film was sputtered by Ar ions for 30 mins at a rate of 45
A/min.
FIG. 10 is complex impedance diagrams of LSCF/YSZ interface
resistance at 1000.degree. C. with and without a CSO oxide film as
prepared in Example 1 interposed between the LSCF and YSZ.
FIG. 11 (a) is the cross-section SEM photomicrograph of a YSZ oxide
film deposited on a porous LSM substrate as prepared in Example
2.
FIG. 11 (b) is the cross-section SEM photomicrograph of a YSZ oxide
film deposited on a dense LSCF substrate as prepared in Example
2.
FIG. 12 is the x-ray diffraction patterns showing the phase
development of YSZ oxide films deposited on a porous LSM substrate
as prepared in Example 2.
FIG. 13 is the x-ray diffraction patterns showing the phase
development and reaction of YSZ oxide films deposited on a dense
LSCF substrate as prepared in Example 2.
FIG. 14 (a) is a surface SEM of a LSCF oxide film deposited on a
YSZ substrate after four spin-coatings and annealing at 600.degree.
C. for 2 hrs as prepared in Example 4.
FIG. 14 (b) is a cross-section SEM of a LSCF oxide film deposited
on a YSZ substrate after four spin-coatings and annealing at
600.degree. C. for 2 hrs as prepared in Example 4.
FIG. 15 is a graph of interfacial resistance (Ri) versus
temperature (.degree.C.) for various metal oxide systems.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, a method is provided for
preparing substrates coated with a thin, dense polycrystalline
metal oxide film at relatively low processing temperatures. The
process of the present invention may be used, for example, to
produce components for intermediate temperature SOFCs or gas
separation membranes.
The method of the present invention can be used to prepare
substrates coated with thin films of a wide variety of
polycrystalline metal oxides, including films comprised of
"complex" metal oxides (i.e., oxides containing more than one
cation constituent). In addition to YSZ, LSM and LSCF, other
exemplary metal oxides which can be produced as thin films by the
method of the present invention and which have particular
application as components in intermediate temperature SOFC's
include (CeO.sub.2).sub.1-x (SmO.sub.1.5).sub.x (CSO) as well as
perovskite-type oxides, both doped (e.g., La.sub.1-x Mg.sub.x
CrO.sub.3) or undoped (e.g., LaCrO.sub.3). In the preceding
formulas, x has a value between 0 and 1 and can be varied to
provide an oxide of the desired nominal composition. CSO oxide
films may be used as a buffer layer in SOFCs to reduce interactions
between the electrolyte and cathode while La.sub.1-x Mg.sub.x
CrO.sub.3 may be used as interconnect material. In addition to SOFC
applications, LSCF oxide films may be used as gas separation
membranes. Other metal oxides which can be produced as thin films
by the method of the present invention include: NiO, MgO, Al.sub.2
O.sub.3, CaO, SrO, BaO, TiO.sub.2, Cr.sub.2 O.sub.3, MnO.sub.2,
Fe.sub.2 O.sub.3, CuO, ZnO, Y.sub.2 O.sub.3, ZrO.sub.2, Nb.sub.2
O.sub.5, SnO.sub.2, LaO.sub.3, CeO.sub.2, Sm.sub.2 O.sub.3 and
combinations thereof.
For oxide films used in SOFC applications, the value of x is
preferably between 0.02 and 0.10 for (ZrO.sub.2).sub.1-x
(YO.sub.1.5).sub.x (YSZ), between 0.05 and 0.50 for La.sub.1-x
Sr.sub.x MnO.sub.3 (LSM) and between 0.05 and 0.20 for
(CeO.sub.2).sub.1-x (SmO.sub.1.5).sub.x (CSO). Moreover, for oxide
compositions of La.sub.1-x Sr.sub.x Co.sub.y Fe.sub.1-y O.sub.3
(LSCF) used in SOFC applications, the value of x is preferably
between 0.1 and 0.8 and the value of y is preferably between 0 and
1.
The substrates on which the metal oxide films are deposited will
vary depending upon the intended application of the oxide-coated
substrate. In SOFC applications, both the substrate and the oxide
film itself typically serve as a component of the SOFC. For
example, a film of LSM or LSCF may be deposited onto a substrate
comprised of YSZ. In the resulting structure, the LSM or LSCF film
serves as an electrode while the YSZ substrate serves as an
electrolyte. Optionally, a film of YSZ may be deposited onto an LSM
or LSCF substrate. Thus, when the coated substrate is intended to
be used in SOFC applications, both the material selected to
comprise the substrate as well as the oxide film itself should be
compatible with their intended function in the SOFC.
In gas separation applications, the substrate on which the metal
oxide membrane is deposited must be sufficiently porous to allow
transport of the selected gas component across the
substrate/membrane boundary and should not react with the metal
oxide membrane under operating conditions. Typically, the substrate
used in gas separation applications has an average pore diameter in
excess of about 5 .mu.m. In any event, regardless of the intended
use of the oxide-coated substrate, by controlling various process
parameters as will be described herein, the porosity of the
substrate may vary considerably and satisfactory results still
achieved.
Referring to FIG. 1, a flow chart for the film synthesis method of
the present invention is shown. Generally, the method comprises:
(1) preparing a precipitate-free starting solution containing
cations of the desired oxide's metal constituents dissolved in an
aqueous mixture comprising a polymerizable organic solvent (e.g.,
ethylene glycol); (2) heating the starting solution to form a
polymeric precursor free of precipitates and comprising a polymer
containing the metal cations; (3) depositing a thin film of the
polymeric precursor onto a substrate using a spin-coating
technique; and (4) calcining the deposited film of polymeric
precursor in the presence of oxygen to convert the film of
polymeric precursor into the polycrystalline metal oxide film.
The starting solution is prepared by dissolving a source compound
for cations of the oxide's metal constituents in an aqueous mixture
comprising a polymerizable organic solvent. Suitable organic
solvents include those having carbonyl functional groups capable of
polymerization. Preferably, the organic solvent is ethylene glycol.
The cation source compounds suitable for use in this invention are
those which exhibit substantial solubility in aqueous solutions and
include nitrates, chlorides, carbonates, alkoxides and hydroxides
of the appropriate metals in addition to the metals themselves.
Preferably, the cation source compounds are nitrates, chlorides or
carbonates, either hydrated or anhydrous, since these compounds are
relatively inexpensive, easily accessible, and readily soluble in
aqueous solutions. Preferably, the cation source compounds are
first standardized thermogravimetrically in order to confirm their
actual metal content and ensure that the final oxide will have the
desired nominal composition. After standardization, appropriate
amounts of the cation source compounds are completely dissolved in
the starting solution, the amount dissolved being calculated on the
basis of the desired nominal composition of the oxide and the total
amount of the oxide to be prepared.
The starting solution is then heated to expel water and other
volatile components of the starting solution and form a viscous
polymeric precursor comprising a polymer containing the metal
cations. It is critical that the cations remain in solution
throughout the polymerization process. The formation of
precipitates may lead to inhomogeneities and a nonuniform metal
distribution in the resulting oxide as well as lead to the
formation of cracks or pinholes in the oxide film. Precipitation is
prevented by controlling the pH of the starting solution. The
specific pH range of a starting solution which will prevent
precipitation upon polymerization is dependant upon the particular
metal oxide system and may be determined experimentally. This can
be done by preparing several samples of the starting solution for a
particular metal oxide system, each sample varying incrementally in
pH, and then observing which starting solution(s) yield a
precipitate-free precursor upon subsequent heating.
The pH of the starting solution can be varied, for example, by
adding a neutral, acidic or basic pH control agent to the starting
solution. An example of a suitable neutral pH control agent is
glycine. Examples of suitable acidic pH control agents include:
nitric acid, hydrochloric acid, citric acid and oxalic acid.
Examples of suitable basic pH control agents include: ammonium
hydroxide and ethylene diamine. Although citric acid and ethylene
diamine may be added to the starting solution to control pH, these
two pH control agents are less preferred because they are believed
to promote crosslinking in the polymeric precursor which may lead
to nonuniform shrinkage of the film upon subsequent heat treatment
and result in cracking of the oxide film.
Table 1 identifies a suitable pH control agent and a pH range for
the starting solution of several oxide systems which will provide a
precipitate-free polymeric precursor upon subsequent heating.
______________________________________ Oxide System pH Range pH
Control Agent ______________________________________ La.sub.1-x
Sr.sub.x MnO.sub.3 pH < 2 HNO.sub.3 (CeO.sub.2).sub.1-x
(SmO.sub.1.5).sub.x 1 < pH < 7 HNO.sub.3 or glycine
(ZrO.sub.2).sub.1-x (YO.sub.1.5).sub.x 6 < pH < 7 glycine
La.sub.1-x Sr.sub.x Co.sub.y Fe.sub.1-y O.sub.3 6 < pH < 7
glycine NiO pH < 2 HNO.sub.3 MgO pH < 2 HNO.sub.3
______________________________________
Once the precipitate-free starting solution has been formed, the
solution is heated by any convenient means, such as by placing the
vessel containing the starting solution onto a hot plate. The
starting solution is heated to a temperature between about
25.degree. and about 100.degree. C., and preferably between about
60.degree. and about 80.degree. C., until the desired viscosity of
the polymeric precursor is obtained. In order to ensure uniform
heating, the starting solution is preferably stirred while heating.
Depending upon the quantity of precursor being prepared,
polymerization and formation of the polymeric precursor having the
desired viscosity typically takes several hours of heating.
Controlling the viscosity of the polymeric precursor is important
in order to obtain high quality oxide films. In general, precursors
with a viscosity below about 50 centipoise (cP) at 25.degree. C. as
measured on a Brooksfield viscometer, Model No. DVII will not
uniformly wet a smooth substrate such as a silicon wafer. On the
other hand, highly viscous solutions having a viscosity greater
than about 500 cP at 25.degree. C. produce inhomogeneous films and
lead to possible crack formation unless the substrate is heated to
higher temperatures. In order to produce high quality films, the
viscosity of the precursor should be between about 50 and about 500
cP at 25.degree. C., and preferably between about 90 and about 190
cP at 25.degree. C. The viscosity of the polymeric precursor is
adjusted by controlling the heating time and temperature of the
starting solution. Generally, the longer the heating time and the
higher the heating temperature, the higher will be the viscosity of
the polymeric precursor.
The structure of the polymer formed upon heating the starting
solution and the polymerization mechanism for a CSO oxide system
having a nominal composition of (CeO.sub.2).sub.0.8
(SmO.sub.1.5).sub.0.2 was investigated using Fourier transform
infrared spectroscopy (FTIR). The starting solution for the CSO
system analyzed was formed by dissolving appropriate quantities of
standardized reagent grade Ce(NO.sub.3).sub.3.6H.sub.2 O and
Sm(NO.sub.3).sub.3.6H.sub.2 O in an aqueous mixture of 10 ml
distilled water, 40 ml ethylene glycol and 10 ml concentrated
nitric acid. The FTIR spectra of the polymeric precursor for the
CSO oxide system was taken as a function of starting solution
heating time and compared to the FTIR spectra of ethylene glycol.
FIG. 2 shows the FTIR transmission spectra of ethylene glycol and
the polymeric precursor for (CeO.sub.2).sub.0.8
(SmO.sub.1.5).sub.0.2 after heating the starting solution for 24,
48 and 72 hours at a temperature of about 80.degree. C. By
identifying changes in molecular structure of the precursor during
heating relative to ethylene glycol, the mechanism of the
polymerization process and a proposed structure of the polymer for
the CSO oxide system was determined. The results, shown in FIG. 2,
suggest the following structural changes upon heating of the
polymeric precursor:
1. Strong absorption at about 1600 cm.sup.-1 indicates the presence
of C.dbd.O groups after 24 hours of heating time. The absorption
intensity increases with increasing heating time.
2. The strong absorption in the 1338 cm.sup.-1 range is
attributable to the stretching mode of the O--C.dbd.O group after
24 hours of heating time.
3. The decrease in intensity of the C--H stretching (2950 to 2880
cm.sup.-1) indicates the oxidation of ethylene glycol.
4. The new absorption bands in the range of 770 cm.sup.-1 after 24
hours heating time indicates the existence of oxalate ions.
5. The weak absorption bands observed in the range of 330 to 650
cm.sup.-1, indicates the presence of metal-oxygen bonds in the
structure.
Although the present invention is not limited to such a theory, the
results of the FTIR investigation suggest that the structure of the
polymer for the CSO system contains ethylene glycol and oxalate ion
groups. The metal cations are chelated to the oxalate ion groups to
form a coordination complex. The proposed polymerization mechanism
for the CSO system as well as the resulting polymeric unit are
shown below. ##STR1## As can be seen, it is believed that in this
CSO system, the nitric acid present in the starting solution
oxidizes the ethylene glycol and converts it to oxalic acid which
then forms a polyester having a metal cation chelated to the
deprotonated carboxylic groups. This process involves considerably
less three-dimensional cross-linking as compared to previous liquid
mix methods. Accordingly, the polymer formed is believed to be
substantially linear, providing relatively uniform shrinkage upon
subsequent heat treatment of the deposited film. Specifically, the
method of the present invention is believed to provide a polymeric
precursor such that shrinkage of the film of polymeric precursor
deposited on the substrate occurs primarily in the direction
perpendicular to the surface of the substrate upon subsequent
calcination due to the substantially linear nature of the polymeric
precursor.
Once the polymeric precursor has been formed, a film of the
precursor is deposited on the substrate using a conventional
spin-coating technique. First, a sufficient quantity of the
precursor is placed on the substrate. The substrate is then spun,
such that centrifugal forces spread the precursor across the
surface of the substrate, thereby coating the substrate with a thin
film of the precursor. Suitable spin-coating apparatus for use in
the method of the present invention include those available from
Brewer Science, Inc., Rolla, Mo. 65401, such as Model Nos. 100 and
100CB.
By controlling the viscosity of the precursor, the spinning speed
and the spinning time, the thickness of the oxide film obtained
upon subsequent heat treatment may be controlled. Furthermore, by
controlling these various parameters, the polymeric precursors can
be deposited uniformly on substrates of varying porosity.
Generally, thicker films are produced by increasing the viscosity
of the precursor and/or decreasing the spinning speed and/or
spinning time. Also, as the porosity of the substrate increases, it
is generally desirable to increase the viscosity and/or decrease
the spinning speed and spinning time. For precursors having a
viscosity between about 90 and 190 cP at 25.degree. C., the
substrate spinning rate is typically between about 1500 and about
3000 rpm.
Preferably, the viscosity of the precursor and the spinning of the
substrate are controlled such that the thickness of the oxide film
obtained from a single spin-coating does not exceed about 0.5 .mu.m
and, preferably does not exceed about 0.3 .mu.m. A maximum oxide
film thickness not in excess of about 0.5 .mu.m is preferred
because there is a greater tendency for gaseous volatiles to be
trapped within thicker oxide films obtained from a single
spin-coating. Trapped gases form blisters or bubbles within the
oxide film when the film of polymeric precursor deposited on the
substrate is subjected to calcination. This results in cracking of
the oxide film as well as poor adhesion between the oxide film and
the substrate. If oxide films thicker than 0.5 .mu.m are desired,
it is preferred that they be formed using multiple spin-coatings as
described below.
Once a thin film of the precursor has been deposited on the
substrate by spin-coating, the as-deposited film is thermally
converted into the desired oxide by heat treating the coated
substrate. Heat treating the coated substrate is preferably
conducted in several steps in order to ensure a dense film is
formed. First, the coated substrate is dried at a temperature of
about 80.degree. C. for a time sufficient to evaporate any
remaining solvent from the precursor. Drying the coated substrate
at relatively low temperatures to remove solvent prior to
subjecting the coated substrate to higher calcining temperatures
tends to reduce the formation of gaseous volatiles which may cause
the oxide film to crack. Drying of the deposited film can be
carried out using any suitable heating apparatus such as a hot
plate, laboratory oven or infra red lamp.
Once dried, the coated substrate is heated to a temperature not in
excess of about 300.degree. C. and preferably not in excess of
about 250.degree. C. for a time sufficient to burn-out the organic
components of the film and convert the coating on the substrate
into a substantially solid, amorphous oxide. Formation of the
polycrystalline microstructure and grain growth are achieved by
calcining the amorphous film in the presence of oxygen at
temperatures not in excess of about 600.degree. C., preferably in
the range of about 300.degree. to about 600.degree. C. Calcining is
preferably discontinued prior to the average grain size exceeding
about 0.5 .mu.m, and preferably prior to the average grain size
exceeding about 0.2 .mu.m. Calcining temperatures above about
600.degree. C. tend to cause unwanted thermal interaction between
the film and substrate as well as rapid grain growth. Furthermore,
it is believed that an average grain size above about 0.5 .mu.m
leads to voiding (i.e., crack formation) in polycrystalline oxide
films having a thickness less than about 0.5 .mu.m.
If an oxide film having a thickness greater than about 0.5 .mu.m is
desired, the above-described method is modified in the following
manner. Once the first deposited film of the precursor has been
converted into a solid, amorphous oxide by drying and heating,
successive films of the amorphous oxide are applied by depositing
additional precursor on the coated substrate, spinning the
substrate and then drying and heating each additional layer until a
composite film of the desired overall thickness has been deposited
on the substrate. Once the desired thickness has been obtained, the
film is calcined as described above to produce the polycrystalline
metal oxide. The thickness of each successive, amorphous oxide film
deposited on the substrate preferably does not exceed 0.5
.mu.m.
The present invention is illustrated by the following examples
which are merely for the purpose of illustration and are not to be
regarded as limiting the scope of the invention or manner in which
it may be practiced.
EXAMPLE I
CSO Thin Film
Preparation of the Precursor Solution
A starting solution with a nominal Ce:Sm composition of 0.8:0.2
(molar ratio) was prepared using reagent grade Ce(NO.sub.3).sub.3
6H.sub.2 O and Sm(NO.sub.3).sub.3 6H.sub.2 O as cation source
compounds. These materials were standardized thermogravimetrically
to confirm the actual cation contents. Appropriate quantities of
these materials to include in the starting solution were then
calculated on the basis of obtaining 0.02 mole of the oxide having
the desired nominal composition. Measured amounts of the cation
source compounds were then mixed with 10 ml distilled water, 40 ml
ethylene glycol and 10 ml concentrated nitric acid in a 100 ml
beaker to form a precipitate-free starting solution. The starting
solution was heated on a hot plate at about 80.degree. C. to expel
the water and other volatile matter until it turned to a viscous
liquid.
The change in the viscosity of the solution due to polymerization
was measured at room temperature by means of a Brookfield
viscometer, Model No DVII. FIG. 3 shows the change in viscosity of
the polymeric precursor as function of heating time. The viscosity
of the precursor increases significantly with increased heating
time due to the increase in average molecular weight as a result of
polymerization.
Deposition and Formation of the Dense Films
A spin-coating technique was used to form wet films of the
precursor on various substrates. A few drops of the viscous
precursor were deposited onto the substrate which was fixed on a
spinning disk. The film thickness was established by controlling
the spinning speed, the spinning time, and the viscosity of the
precursor. Generally, the thickness of the deposited films
increased with increasing viscosity of the precursor and decreasing
the spinning speed and time. In general, precursors with viscosity
below 50 cp at 25.degree. C. could not homogeneously wet a smooth
substrate, such as a Si wafer or a glass. On the other hand, highly
viscous precursors having a viscosity above about 500 cP at
25.degree. C. resulted in inhomogeneous films and crack formation
unless the substrate was heated at higher temperatures. Therefore,
it is important to control the viscosity of the solution to obtain
high quality films. The viscosity of the precursor solutions used
in this Example ranged between 90 and 190 cP at 25.degree. C. With
a spinning speed of between about 2000 and about 3000 rpm, dense
oxide films were obtained having a thickness in the range of about
0.1 to about 0.3 .mu.m for each coating after firing at about
600.degree. C.
After spin-coating, the as-deposited films of the precursor were
dried on a hot plate at about 80.degree. C. for 1 minute then
placed directly on a preheated hot plate at about 320.degree. C. or
in a tube-furnace at about 400.degree. to 800.degree. C. for 1
minute and quenched in air. The sample temperature was monitored by
a thermocouple. Thicker films were produced by multiple coatings
with drying and heat-treatment after each coating. No
inhomogeneities were observed due to multiple spin-coatings.
To decrease the processing time and the interdiffusion between the
CSO oxide film and the substrate, a rapid thermal annealing (fast
firing) process was adopted. FIGS. 4 (a)-(d) show the 3-D atomic
force microscopy (AFM) micrographs for the CSO oxide film deposited
on a Si substrate. The oxide films shown in FIGS. 4 (a)-(d) were
prepared from a precursor having a viscosity of about 90 cP at
25.degree. C. and using a spinning speed of about 2500 rpm. The
as-deposited films were then fast fired for 1 minute at
320.degree., 400.degree., 600.degree., and 800.degree. C.,
respectively. The films appear to be smooth and crack-free with a
grain size of about 0.1 .mu.m at firing temperatures below
600.degree. C. Some microcracks can be observed in the film heated
at 800.degree. C. (See FIG. 4 (d)). Accordingly, the fast firing
process for making dense CSO films is suggested at temperatures
below 600.degree. C.
FIG. 5 shows a cross-sectional SEM micrograph of a CSO oxide film
deposited on a LSCF substrate after five spin-coatings and fast
firing at 600.degree. C. for 1 min. The deposited film is 0.5 .mu.m
in thickness and appears to be highly dense and uniform.
Characterization of the CSO films
The phase development of CSO oxide films deposited on Pt and LSCF
substrates were studied by x-ray diffraction analysis performed on
a Scintag diffractimeter, using Cu Ka radiation. The results are
shown in FIGS. 6 and 7, respectively. The formation of the cubic
fluorite structure is observed at temperatures as low as
320.degree. C. on both Pt and LSCF substrates. It is not uncommon
for Pt and LSCF, both possessing good catalytic properties, to act
as nucleating sites, allowing CSO nuclei to form on the surface at
relatively lower temperatures and thus accelerating the
crystallization process of CSO. Similar behavior has been observed
for PZT films on Pt substrates. The absence of other impurity or
nonstoichiometric phases shows that the composition of the film is
as intended. FIGS. 6 and 7 also show that the CSO film is not
reactive with either Pt or LSCF substrates up to 1000.degree.
C.
A PHI 545 Auger electron spectroscope (AES) was used to study the
surface and bulk composition of a CSO oxide film deposited on a Si
substrate. A surface Auger spectrum of the film, after annealing at
600.degree. C. for 30 minutes, is shown in FIG. 8. Only Ce, Sm, and
O atoms were detected with a small amount of carbon and argon
(about the detection limit of the AES). This shows that the purity
of the CSO oxide films is fairly high.
The in-depth AES compositional profile for the CSO oxide film
deposited on a Si substrate is shown in FIG. 9. The film was
sputtered by Ar ions for 30 mins at a rate of 45 A/min. FIG. 9
shows a fairly constant Ce/Sm atomic ratio of about 4:1 extending
to a depth of about 0.1.mu.m from the surface of the oxide film.
This indicates that our process provides good stoichiometric
control and oxide composition homogeneity. A well-defined
film/substrate interface is also observed, indicating that no
significant interdiffusion between the CSO film and the Si
substrate occurred.
AC impedance spectroscopy was used to study the LSCF/YSZ
interfacial resistance with and without a CSO buffer layer. These
studies were carried out by a two-electrode method (two
symmetric-electrode cell) using a Schlumberger 1260 impedance
analyzer. Pt mesh, pressed to the sample by a spring, was used as
the current collector, a small ac signal (<10 mV) was employed,
without a dc bias voltage. A trilayer of LSCF/CSO/YSZ was made by
first depositing a CSO oxide film (approximately 0.1 .mu.m thick)
onto a YSZ substrate by multiple spin-coatings and then slurry
coating a porous LSCF electrode onto the CSO oxide film. This
structure was then heated to 1100.degree. C. for 2 h. The developed
trilayer was tested and compared with the LSCF/YSZ system without a
CSO oxide layer. FIG. 10 shows the complex impedance diagrams of
LSCF/YSZ interfacial resistance at 1000.degree. C., with and
without a CSO oxide film interposed between LSCF and YSZ as a
buffer layer. With the frequency sweep, equivalent electrical
circuits of the electrode processes were obtained and the
components resolved, thus identifying the electrode/electrolyte
interfacial resistance. The interfacial resistance was calculated
from the difference of the real-axis intercepts of the semicircle.
The larger the semicircle the higher the interfacial resistance,
and lower the cell performance. The intercept of the real-axis, at
the high frequency end of the semicircle, gives the bulk resistance
of the electrolyte. FIG. 10 shows that upon incorporation of the
CSO oxide film between LSCF and YSZ, a significant reduction in
interfacial resistance (small semicircle) was observed. It also
shows that the bulk resistance of the electrolyte did not change
after applying the CSO buffer layer. Thus, CSO can serve as a
protective layer impeding the interactions between LSCF and YSZ
that were previously observed and resulted in the formation of
LaZr.sub.2 O.sub.7 and SrZrO.sub.3 that dramatically increased
interfacial resistance. Interaction studies on LSCF/CSO, LSM/CSO,
and YSZ/CSO bilayer systems showed that no reaction products are
formed at the interface at temperatures up to 1200.degree. C.
Accordingly, the presence of CSO buffer between LSCF and YSZ will
eliminate the possibility of second-phase formation and will
improve cell performance at temperatures up to 1000.degree. C.
EXAMPLE II
YSZ Thin Film
Preparation of the Precursor Solution
A starting solution with a nominal Zr:Y composition of 0.84:0.16
(molar ratio) was prepared using reagent grade Zirconyl Chloride
hydrate (ZrOCl.sub.2.8H.sub.2 O) and Yttrium nitrate hydrate
(Y(NO.sub.3)3.6H.sub.2 O). These materials were standardized
thermogravimetrically to confirm the actual cation contents.
Appropriate quantities of these materials to include in the
starting solution were then calculated on the basis of obtaining
0.02 mole of the oxide having the desired nominal composition.
Measured quantities of the cation source compounds were then mixed
with distilled water (20 ml), ethylene glycol (40 ml) and glycine
(0.02 mole) in a 100 ml beaker to form a precipitate-free starting
solution. The starting solution was then heated on a hot plate at
about 80.degree. C. to expel the water and other volatile matter
until it turned to a viscous liquid. The change in the viscosity of
the solution as it was converted into the polymeric precursor was
measured at room temperature by means of a Brookfield viscometer,
Model No. DVII.
Deposition and Formation of the Dense Film
A spin-coating technique was used to deposit wet films of the
precursor on porous LSM and dense LSCF substrates. In depositing a
wet film of the precursor on a dense LSCF substrate, a precursor
viscosity of about 90 cP at 25.degree. C. was employed along with a
spinning rate of about 2500 rpm for 20 seconds. The as-deposited
films of the YSZ precursor were dried by placing the coated
substrate on a hot plate at about 80.degree. C. The dried films
were then heat treated at about 320.degree. C. to remove most of
organic content and obtain a 0.1 .mu.m thick film of the amorphous
oxide. The process was repeated with successive spin-coatings of
the precursor with drying and heating treatment after each coating
until a continuous film of the desired thickness was obtained. To
obtain a continuous film of YSZ on a porous LSM substrate the
precursor viscosity was increased to about 190 cP at 25.degree. C.
and multiple depositions were employed. Crystallization and grain
growth of the as deposited amorphous films were obtained through
subsequent heat treatment. FIGS. 11 (a) and 11 (b) show the
cross-section SEM photomicrographs of the YSZ oxide films deposited
on porous LSM and dense LSCF substrates after 14 spin-coating
cycles and 5 spin-coating cycles, respectively. A dense continuous
YSZ oxide film of about 0.8 .mu.m thickness was successfully
deposited on the porous LSM substrate.
Characterization of the Films
The structural evolution of the as-deposited amorphous YSZ films
during annealing at temperatures between 600.degree. C. and
1200.degree. C. was monitored by x-ray diffraction. The results for
YSZ oxide films deposited on porous LSM substrates and for YSZ
films deposited on dense LSCF substrates are shown in FIGS. 12 and
13 respectively. As shown in FIG. 12, fine-grain polycrystalline
YSZ was detected on the porous LSM substrates at annealing
temperature as low as 600.degree. C. Grain growth of the YSZ oxide
films was observed at higher temperatures. However, as shown in
FIG. 13, when YSZ films were deposited on a dense LSCF substrate a
reaction was detected at temperatures as low as 1000.degree. C.
This was eliminated by depositing a dense CSO oxide film as a
buffer layer, as previously described, between the LSCF substrate
and the YSZ oxide film. X-ray diffraction results show that CSO
film is compatible with the LSCF cathode material. No reaction
products were detected after annealing a LSCF substrate coated with
a CSO oxide film at 1200.degree. C. for 24 hours. The CSO buffer
layer was found to eliminate the interactions between YSZ and LSCF
at high temperatures and significantly reduced the
cathode/electrolyte interfacial resistance.
EXAMPLE III
LSM Thin Film
Preparation of the Precursor Solution
A starting solution with a nominal La:Sr:Mn composition of
0.85:0.15:1 (molar ratio) was prepared using reagent grade La.sub.2
(CO.sub.3).sub.3.H.sub.2 O, SrCO.sub.3 and MnCO.sub.3 as cation
source compounds. These materials were standardized
thermogravimetrically to confirm the actual cation contents.
Appropriate quantities of these materials to include in the
starting solution were calculated on the basis of obtaining 0.01
mole of the oxide having the desired nominal composition. Measured
quantities of the cation source compounds were then mixed with 10
ml of distilled water and 20 ml of ethylene glycol in a 100 ml
beaker. The carbonate cation source compounds were dissolved by the
slow addition of 5 ml of concentrated nitric acid to the beaker to
form a precipitate-free starting solution. The starting solution
was heated on a hot plate at about 80.degree. C. to expel the water
and other volatile matter until it turned into a viscous liquid.
The change in the viscosity of the solution as it was converted
into the polymeric precursor was measured at room temperature by
means of a Brookfield viscometer, Model No. DVII.
Deposition and Formation of the Dense Film
The spin-coating technique was used to form wet films of the LSM
precursor on a dense YSZ substrate. In order to obtain a dense LSM
oxide film 0.3 .mu.m thick on a dense YSZ substrate, a precursor
viscosity of about 190 cP at 25.degree. C. and a spinning speed of
about 2500 rpm for 20 seconds were employed.
Characterization of the Films
X-ray diffraction analysis of the LSM oxide film calcined at
600.degree. C. showed only LSM and YSZ phases. A reaction between
the LSM oxide film and the YSZ substrate occurred at 1050.degree.
C. forming La.sub.2 Zr.sub.2 O.sub.7 at the interface. This could
be a source for cathodic polarization and reduced cell efficiency.
A dense CSO film was successfully deposited on a dense LSM
substrate. X-ray diffraction results show that CSO oxide films are
compatible with the LSM cathode material. No reaction products were
detected after annealing the LSM/CSO system at 1200.degree. C. for
24 hours. A CSO oxide film interposed between YSZ and LSM as a
buffer layer has been found to eliminate the interactions between
YSZ and LSM at high temperature.
EXAMPLE IV
LSCF Thin Film
Preparation of the Precursor Solution
A starting solution with a nominal La:Sr:Co:Fe composition of
0.6:0.4:0.2:0.8 (molar ratio) was prepared using reagent grade
La.sub.2 (CO.sub.3).sub.3.xH.sub.2 O, SrCO.sub.3,
Co(CO.sub.3).xH.sub.2 O and Fe(NO.sub.3).sub.3.xH.sub.2 O as cation
source compounds. These materials were standardized
thermogravimetrically to confirm the actual cation contents.
Appropriate quantities of these materials to include in the
starting solution were then calculated on the basis of obtaining
0.02 mole of the oxide having the desired nominal composition.
Measured quantities of the cation source compounds were then mixed
with 20 ml of distilled water and 40 ml ethylene glycol in a 100 ml
beaker. The carbonate cation source compounds were then dissolved
by the slow addition of 10 ml of concentrated nitric acid to the
beaker to form a precipitate-free starting solution. After
dissolution, 0.04 mole glycine was added. The starting solution was
then heated at about 80.degree. C. on a hot plate to expel the
water and other volatile matter until it turned to a viscous
liquid. The change in the viscosity of the solution as it was
converted into the polymeric precursor was measured at room
temperature by means of a Brookfield viscometer, Model No.
DVII.
Deposition and Formation of the Dense Films
A spin-coating technique was used to form wet films of the LSCF
precursor on a YSZ substrate. The film thickness was established by
controlling the spinning speed, the spinning time and the viscosity
of the precursor. In this example, a precursor viscosity of about
150 cP at 25.degree. C., a spinning speed of 3000 rpm for 20
seconds, and an annealing temperature of 600.degree. C. yields an
oxide film about 0.1 .mu.m thick for each coating. Thicker films
were produced by multiple coatings with drying and heat treatment
after each coating. Crystallization and grain growth of the
as-deposited amorphous films were obtained by subsequent heat
treatment. The average time for producing a dense, 0.5 .mu.m thick
film of LSCF on a YSZ substrate can be reduced to 5 to 10 minutes s
by eliminating intermediate heat treatment cycles.
Characterization of the Films
FIGS. 14 (a) and (b), respectively, show typical surface and
cross-section SEM micrographs of LSCF oxide films on a YSZ
substrate after four spin-coatings and annealing at 600.degree. C.
for 2 hours. FIG. 14 (a) shows that the film is dense and free from
microcracks and pinholes. The cross-section SEM micrograph in FIG.
14 (b), shows that the thickness of the LSCF oxide film is about
0.4 .mu.m. The electrode characteristics of the dense LSCF films
were investigated by AC impedance spectroscopy. FIG. 15 shows plots
of the interfacial resistance, Ri, versus temperature for various
oxide systems. The LSCF oxide film electrode deposited on both YSZ
and CSO substrates possessed very low electrode resistance at
temperatures as low as 500.degree. C. In FIG. 15, the interfacial
resistance using porous Pt and dense LSM oxide film electrodes
deposited on different substrates are also shown for purposes of
comparison. Both porous Pt and dense LSM electrodes deposited on
YSZ or CSO substrates showed considerably higher interfacial
resistance as compared to LSCF electrodes at temperatures up to
900.degree. C. These results show that at temperature below
800.degree. C., LSCF oxide film electrodes deposited on either CSO
or YSZ substrates have lower interfacial losses as compared to
either porous PT or LSM electrodes. Accordingly, the LSCF oxide
film compositions are more promising than LSM, or even Pt, for SOFC
cathode applications at temperatures between
600.degree.-800.degree. C.
In view of the above, it will be seen that the several objects of
the invention are achieved.
As various changes could be made in the above methods without
departing from the scope of the invention, it is intended that all
matter contained in the above description be interpreted as
illustrative and not in a limiting sense.
* * * * *